Mass spectrometers are widely used instruments for providing information about the nature and structure of molecules, including large biomolecules such as peptides or proteins. An important component in the construction of a mass spectrometer system is a source for producing ions of the molecule or molecules of interest (i.e., the analyte molecules) to enable subsequent separation and detection by mass spectrometry.
Matrix assisted laser desorption and ionization (MALDI) is one well-known technique for the production of analyte ions. The MALDI process may be conceptualized as having two steps. In a first step, the analyte is mixed with a solvent containing small organic molecules in solution, called a matrix. The matrix is chosen to have a strong absorption at the specific wavelength of a laser used in the second step. The mixture is dried prior to analysis, removing any liquids used in preparation of the solution. The result is a solid deposit of an analyte-doped matrix, where the analyte molecules are embedded throughout the matrix and where the analyte molecules are isolated from each other. In a second step of the MALDI process, intense pulses of the laser are directed at the analyte-doped matrix. The pulses cause ablation of bulk portions of the solid solution. The rapid heating causes localized sublimation of the matrix and expansion of sublimated matrix portions into a gas phase, entraining intact analyte. Ionization reactions occur during or prior to this process and produce the analyte ions, which are subsequently conveyed to a mass analyzer for determination of the mass-to-charge ratios (m/z's) of the analyte ions and/or its products.
The MALDI technique offers important advantages relative to alternative ionization techniques, such as electrospray ionization (ESI), which are tied to the time limitations of the chromatographic separation process. Standard sample preparation methods developed for MALDI, provide for easy storage of prepared samples and enable samples of interest to be re-analyzed at any suitable time. The pulsed operation of MALDI gives an opportunity to look closely into specific compounds without being restricted to analysis the time period defined by an elution peak. These features of MALDI found further development in LC-MALDI technique which breaks chromatographic elution process into a number of short time events frozen as separate samples on a MALDI plate.
Certain limitations in the use of the conventional MALDI technique arise from its inability to produce multiply charged analyte ions. There has been recent interest in utilizing advanced fragmentation techniques based on ion-electron and ion-ion reactions, such as electron capture dissociation (ECD) and electron transfer dissociation (ETD), which are characterized by a significant improvement in efficiency of fragmentation with increased charge state of analyte ions. Furthermore, many commercially available mass analyzers are limited in operation to ions having m/z's within a specified range (e.g., below 3000 Th), rendering analysis of large biomolecules by MALDI-based mass spectrometry difficult or impossible.
One approach to adapting the standard MALDI technique for production of multiply charged ions is described in U.S. Patent Application Publication No. US2005/0199823 by Jochen Franzen. This reference discloses an ion source in which analyte molecules are desorbed from the surface of a solid sample (using a pulsed laser) in close proximity to a spray of charged solvent droplets emanating from a conventional electrospray capillary. A portion of the desorbed analyte molecules are protonized (purportedly by interaction with either the charged droplets or free proton-water complexes vaporized from the droplets) and form multiply-charged analyte ions. While this method appears to be somewhat successful in producing the desired multiply-charged ions, it is believed that ionization efficiencies achieved using this method are highly sensitive to variations in spray conditions (more specifically, the concentration and size dispersion of small, highly-charged droplets near the sample surface and efficiency of ion transport and incorporation into the droplets), and that departures from optimal conditions may have a substantial adverse effect on the production of multiply-charged ions and hence overall mass spectrometer performance.